It wasn't the hundreds of muzzle flashes that exploded from the shooter's automatic rifles that gave away his position.

Nor was it the panicked 911 calls from people reporting the rhythmic thundering of gunfire.

It was the smoke.

As the gunman, identified as Stephen Paddock, set off round after round, gun smoke filled his room on the 32nd floor of the Mandalay Bay hotel, said Randy Sutton, a retired lieutenant with the Las Vegas Metropolitan Police Department, citing police sources.

The Swat team used the alarm triggered by the smoke to zero in on Paddock's position in about 20 minutes - not nearly enough time for a floor-by-floor search of the hotel, which has 3,309 rooms and a 135,000-square-foot casino.

After they located his room, the Swat team members used explosives to get inside, the sheriff's office said. Paddock (64) killed himself before the officers entered, according to Las Vegas Sheriff Joe Lombardo.

Paddock's room had a bird's-eye view of the concert grounds.

"He rained down hell on those people," Sutton said. "Hundreds of rounds. The sheriff said that there were several firearms found in the room. He was extremely well armed. He must have brought plenty more ammunition, already loaded into magazines."

Police believe Paddock had been staying at the Mandalay Bay hotel since Thursday, concealing a cache of automatic weapons.